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Acquisitions (Details) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Business Combinations [Abstract] |  |  |
Reason for Business Acquisitions | The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services businesses to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth. | The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services businesses to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth. |
Pro Forma Impact of Business Acquisitions | not significant |  | not significant

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